Methods of fumigation involve filling a closed space with gaseous chemicals (fumigants) to eliminate harmful organisms. They are mainly applied to imported foodstuffs such as cereals and fruits and vegetables, and timber as a preventive measure against epidemics, as well as to the extermination of bacteria and insects from green house cultivation soil. From the viewpoint of effectiveness and low cost etc., gases containing organohalogen compounds such as methyl bromide, chloropicrin etc. as fumigation components are used in large amounts as the fumigation gases used in these kinds of fumigation methods.
In actual practice, the above-described kind of fumigation gases containing large amounts of organohalogen compounds are released to the atmosphere after use without being first subjected to any treatment to render them harmless. However, in recent years it has become clear that the ozone layer which plays an important role for the earth's environment is being destroyed by the above-mentioned kind of organohalogen compounds, and restrictions on the use and disposal of organohalogen compounds have become severe worldwide. There is therefore a pressing need for the establishment of a method for purifying fumigation exhaust gases by the effective and low cost decomposition of the organohalogen compounds such as methyl bromide and chloropicrin contained in the fumigation exhaust gases.
Methods of effectively decomposing fumigation components in fumigation exhaust gases have been studied for some time, but due to the above-mentioned situation, are now being increasingly studied from various angles. Even though the concentration of fumigation components contained in the fumigation exhaust gases may be in the order of 1 volume percent at the start of their discharge from the cereal fumigation silo, it drops as the discharge is continued to reach a final value in the order of a number of parts per million. It was found to be extremely difficult to effectively decompose and eliminate the fumigation components discharged over such a wide concentration range.
Direct combustion methods, plasma methods, chemical absorption methods, catalytic oxidation methods, and adsorption methods etc. have so far been proposed, but these methods have the following problems.
Firstly, direct combustion methods usually require high temperatures over 600.degree. C., and have the practical problems that running costs such as fuel costs are extremely high, and that extremely harmful halogen gases, such as Cl.sub.2, BR.sub.2, are generated by the combustion of the organohalogen compounds.
Plasma methods have been actively studied in recent years. However, they consume large amounts of electric power, and require highly expensive noble gases such as helium, argon etc. Accordingly, although they can be said to be suited to the treatment of certain special objects, they are unsuitable as methods for purifying large amounts of fumigation exhaust gases.
Furthermore, with respect to chemical absorption methods, special chemicals are required, and large amounts of waste water are generated which require secondary treatment. Also, the elimination efficiency is low and the method is thus impractical.
The method disclosed in Japanese Patent Application Publication No. Hei 3-289973 is one example of a catalyst oxidation method. With this method, although the organohalogen compounds can be oxidatively decomposed by contact with a composite oxide catalyst, there is the fear that large amounts of carbon monoxide will be generated, and that extremely harmful phosphogenes, bromophosphogenes etc. will be generated through reaction of the carbon monoxide with the halogens, with the consequent problem that an oxidation catalyst has to be further provided in an after-stage.
In adsorption methods, activated carbon is generally used as the adsorbent. This adsorbent has the characteristic that when it is used with respect to low concentration ranges, only a small amount of the catalyst need be used, and it has a long breakthrough time. However, in situations wherein the initial discharge concentration is high, such as with fumigation exhaust gases, not only does the concentration of adsorbed component after adsorption become high but the breakthrough time becomes very short, and in order to overcome these problems, it is necessary to use the adsorbent in extremely large amounts.
The present invention was made in light of the prior art described above, and has as its objective the provision of a catalyst for purifying fumigation exhaust gases with which fumigation exhaust gases can be purified economically and efficiently, as well as a method for purifying fumigation exhaust gases employing said catalyst.